606 TRANSACTIONS OF SECTION D. 



It does not seem possible to bvino; the forms of tlie monaxons into any relation 

 •with the crystalline nature of the material composing them, but, on the other 

 hand, all their peculiarities can be correlated -with their position and functions in 

 the sponge body. Unlike a crystal, the monaxons have dissimilar ends, which 

 can be attributed to the fact that one end is embedded in the body-wall, while the 

 other projects into the water. The monaxons are invariably curved so as to pro- 

 ject outwards more vertically from the body-wall, and the free end is frequently 

 barbed ; they are thus rendered more efficient as protective organs. Such charac- 

 ters cannot be explained on any physical or mechanical ground. Hence, as regards 

 the primary spicules, we may agree with Maas that they are the outcome of two 

 distinct activities — an organic- ceUular, determining the form of the spicule, and an 

 inorganic-crystalline, determining only the material. 



In the triradiates, similarly, the rays may be straight or curved in various ways ; 

 and here again it is not apparent how the form of the individual rays can be 

 related with the crystalline structure. On the other hand, with regard to the 

 ano-les at which the rays meet, it is a remarkable fact that in one section of these 

 sponges the triradiates are unfailingly equiangular. The principal rays may have, 

 normally or abnormally, secondary branches on them, which are also set at an 

 angle of 120° with one anot,her or with the main stem. That this regularity is due 

 to an inherent hereditary tendency on the part of the sponge to fix one ray to 

 another at an angle of 120° is negatived by the fact that the rays of the triradiates 

 often meet irregularly in the early stages of growth, but become symmetrical as 

 growth proceeds. Maas's observation that sponges grown in sea water devoid of 

 CaCOj produce organic sclerites with no trace of calcite, appears to show that 

 hereditary tendencies influence the formation of spicules to the extent of producing 

 combinations of three primary monaxons to form a triradiate system ; but I do 

 not gather that the organic sclerites have the regular symmetry of angles. I am 

 forced to the conclusion that here the physical properties of the material play a 

 part in determining the spicular form. 



In the majority of Calcarea there occur sagittal triradiates which approach 

 the T-shape. Here we come upon a remarkable fact, discovered by Mr. Bidder, 

 He found that in equiangular triradiates the optic axis of the crystal is always at 

 right angles to the plane of the rays, but that in the sagittal triradiates the optic 

 axis is always.inclined to the plane of the rays in such a way that, if viewed along 

 the crystalline' optic axis, the spicule would always appear equiangular in the 

 projection, whatever its actual shape might be. Hence the relation of the angles 

 at the junction of the rays to the optic axis is the same in all cases, and deviations 

 from the primitive equiangular condition can only be brought about by tilting the 

 optic axis. 



The conclusion to which I have come is that in primary spicules the form is in 

 no^way dependent upon the physical properties of the material, but is regulated 

 solely by biological conditions ; where, however, primary spicules are joined to 

 form spicular systems, physical properties may exert an influence on the form- 

 of the spicule as a whole by determining the angles at which the rays join. This 

 ao-rees with the results I arrived at with regard to the spicules in tlie Ilexacti- 

 nellida (' Zool. Anzeiger,' 1905). 



2. Sitggestions Jor a inore Systematic Study of Oceanic Plankton. 

 By G. Herbert Fowler, B.A., Ph.D. 



In opening the discussion on plankton the writer sketched the history of its 

 study, with special reference to Hensen's theories on the subject. The modern 

 subdivisions of thefplankton and their composition were outlined, and attention 

 drawn to the two schools of plankton-students of the present day — the one 

 almost physiological, the other more purely zoological. The methods of the 

 former were regarded as far more refined than those of the latter. A number of 

 interesting problems which demand attention on the zoological side were briefly 



